The present invention relates generally to countercounter measure devices, and more particularly, to a jammer cancelling circuit which references targets to the jammer location.
Monopulse radar circuits are frequently used in surveillance systems and in range and angle tracking systems. Such monopulse systems generally are adapted to simultaneously emit microwave energy in four offset, but overlapping, adjacent antenna lobe patterns. Each of these lobe patterns is arranged for the irradiation of a respective spacial quadrant located about the boresight or target tracking axis of the radar. The RF signals received from these four adjacent lobe patterns are combined to form a series of sum and difference signals. The sum and difference signals are then multiplied in a phase sensitive detector to obtain both magnitude and angular error information in three coordinates. This information is then utilized to generate three control voltage for positioning the elevation, azimuth and range tracking servos.
Pulse radars in general, and monopulse radars in particular, are vulnerable to jamming due to their directional beam antenna patterns. Such jamming generally comprises the transmission of a signal in the form of a noise-modulated continuous wave which, when detected in the sidelobes of the directional beam antenna, causes an obliteration of the desired target indications.
The detection and the tracking of targets becomes especially difficult if the jammer shares the radar's mainlobe with the target-to-be-detected. Accordingly, various schemes have been developed in the prior art to eliminate the jamming signal from the radar mainlobe. Generally such schemes are implemented by utilizing a notch filter to place a notch in the radar mainlobe at the location of the jammer. However, the location of a notch in the radar mainlobe, as illustrated in FIG. 1, severely distorts the mainlobe pattern. Such a distorted mainlobe pattern prevents the accurate location of targets. This can be understood by noting that targets are generally detected by sweeping the mainlobe of the radar antenna across the target. Thus, the RF reflections from the target, in essence, trace out the mainlobe pattern of the antenna during the sweep. The target may then be located by bisecting this mainlobe target tracing. However, it can be seen that when a notch is located in the mainlobe of the radar antenna, then symmetry cannot be relied on to estimate the target location with any accuracy.